The present invention relates to an active reception antenna arranged on a non-conductive, flat supporting piece set in a conductive body of a motor vehicle. The antenna is effective up to UHF range of wavelengths.
Antennas of this kind are described for example, in DE publications P 3315458, P 3410415, P 3423205, P 3618452, P 3619704 and P 3719692. In all these prior art antennas, a window pane is employed as the non-conductive, flat supporting surface and is surrounded by a conductive frame to which the ground terminal of a four terminal network is connected by a short conductor. The input of the active four terminal network is situated in an immediate proximity to the connection point of the antenna conductor and is connected thereto by a very short conductor in order to achieve optimum properties of the active antenna. The position of the antenna conductor on or near the window pane and the position of the connection point of the antenna conductor are determined by the requisite properties of the antenna with respect to the desired excitation and polarization action. For this reason, in prior art antennas, for example the antennas listed in the above mentioned German patent publications, it has been necessary to provide, in the immediate proximity of the connection point of the antenna conductor, both an installation space for the amplifier and the possibility to connect the amplifier to a ground point of the vehicle body.
The output of the amplifier, which is situated immediately at the ground point, has formed a connector contact point of the antenna from which mostly a coaxial transmission line has led to a receiver. In practice the forms of antennas of this kind must be selected under the consideration of limiting aspects given in the motor vehicle environment, inasmuch as the required proximity to the ground point can be achieved only at limited number of locations. Therefore, many forms of antennas, which in spite of a good performance of their antenna conductor, cannot frequently be realized in practice. For example, if an optimum connection point of the antenna conductor of an antenna structure lies in the range of an edge between the roof of a motor vehicle and the front or the rear window pane, then a matching network or the amplifier must be installed in the range of the roof. In many motor vehicles, the inner side of the roof is covered by a web of fabric which precludes an access to the metal sheet and prevents the installation of the amplifier between the metal sheet and the fabric web.
Under the plastic screens or shields there is also frequently insufficient space for installing components having a minute structural height. Therefore, ground points in the range of deposition racks must have been used in spite of the fact that the antenna structures were situated in the upper region of the window pane of the motor vehicle.
Moreover, by introducing plastic structural parts in the construction of a motor vehicle, such as for example plastic frames surrounding the window of rear trap door of a station wagon, there is no longer the possibility to provide a sufficiently short connection conductor between a ground point of the vehicle body and the connection point of the antenna conductor secured on the pane.
The above described circumstances made it necessary to place an amplifier, which in the following description will be called an active four terminal network, on the vehicle body at a relatively large distance from the connection point of the antenna conductor in order to insure a short connection to the ground point. Consequently, the clearance between the connection point of the antenna conductor on the pane and the input of the four terminal network is to be bridged by a correspondingly long connection wire.
The connection wire is printed on the pane parallel to an edge of the latter or is laid along the upper surface of the vehicle body. In the case of plastic vehicle parts which surround the window pane, the connection wire can be laid also on the upper surface of a plastic part or can be embedded in the plastic part provided that losses of the plastic material or the respective frequency ranges are sufficiently low. However, plastic materials hitherto used in motor vehicles exhibit at frequencies of the ultrashort wavelength range high dielectric losses. Consequently, signals in connection wires which extend in the proximity of the upper surface of such a plastic material are subject to cause high damping and the performance of antennas installed according to the prior art frequently does not meet the requisite standard.
Therefore, the principle of designing an active antenna having a shortest possible conductor between the four terminal network and the connection point of the antenna conductor situated on the window pane, cannot be realized such as to obtain the advantage of the maximum possible signal-to-noise ratio. This disadvantage is present substantially for all frequency ranges, but is particularly serious in the case of relatively low frequencies of the long medium and shortwave ranges for which an antenna amplifier having a capacitive high impedance input is employed. In these frequency ranges the use of a long connection wire brings about the disadvantage of an additional capacitance relative to the vehicle body. The additional capacitance has the disadvantageous effect particularly in the case of electrically short antennas defining a correspondingly small antenna capacity.